Balanced oscillator and transmitter arrangement

ABSTRACT

The present invention teaches an oscillator and transmitter. The balanced oscillator comprises a resonator for generating a reference signal having a resonating frequency, a first oscillator for providing a first oscillating output in response to the resonating frequency, and a second oscillator for providing a second oscillating output in response to the resonating frequency. The second oscillating output has a magnitude equal to the first oscillating output while oscillating 180 degrees out of phase with the first oscillating output. The transmitter comprises an antenna for radiating the first and second oscillating output signals.

RELATED APPLICATIONS

This application is a continuation in part (CIP) application of aapplication Ser. No. 08/342,721, filed on Nov. 21, 1994, now U.S. Pat.No. 5,486,793.

FIELD OF THE INVENTION

This invention relates generally to radio frequency ("RF") transmittersand, more particularly, to a balanced oscillator and transmitter circuitfor radiating RF signals with enhanced power output.

BACKGROUND OF THE INVENTION

Compact radio frequency ("RF") transmitters are widely employed inconnection with remote signal communication systems. Compacttransmitters are commonly used for remotely controlling automatic garagedoor systems, electronic sound systems, televisions and VCRs. In theautomotive industry, compact transmitters are commonly used in remotekeyless entry systems to provide remote control access to a vehicle, aswell as controlling other vehicular functions such as alarm systemfeatures, trunk release, for example. Ideally, compact hand heldtransmitters are battery operated, energy efficient and intended toaccommodate a compact enclosure.

In one known compact remote system design, the transmitter radiates anRF signal with a predetermined carrier frequency encoded according to anon/off switched pattern. This radiating signal is subsequently receivedby a remote receiver. Once received, the signal is processed, ifnecessary, and then provided as a control signal to control a functionor feature of the system.

Currently, a number of compact remote RF transmitters employ a singleoscillator design for providing a local oscillation signal. Asillustrated in FIG. 1, a conventional transmitter circuit 5 is shownwith a single oscillating circuit commonly referred to as the Colpittsoscillator. Transmitter circuit 5 generates a local oscillation signalwhich is transmitted from an antenna element L₁. In light of itssimplicity, circuit 5 has been the transmitter component of choice inautomotive, remote controlled, keyless entry systems.

Referring to FIG. 1 in greater detail, the Colpitts oscillator ofcircuit 5 comprises a Colpitts configured transistor Q₁ and an inputresonant tank circuit. The tank circuit typically comprises a resonator,such as a surface acoustic wave ("SAW") device 2, and a pair of feedbackcapacitors, C₁ and C₂. Further, the oscillator also includes a number ofbiasing resistors to facilitate the proper operation of transistor Q₁.Transmitter circuit 5 also comprises an inductor L₁ which acts as anantenna element for radiating the RF output signal.

Structurally, transistor Q₁ comprises a base 4, collector 6 and emitter8. Base terminal 4 is coupled with surface acoustic wave resonator 2,and collector 6 is coupled with inductor L₁, while emitter 8 is coupledto ground through a resistor R₃. Additionally, feedback capacitor C₂ iscoupled between emitter 8 and ground, and as such, is in parallel withresistor R₃. Feedback capacitor C₁ is coupled between collector 6 andemitter 8. Moreover, a third capacitor C₃ is coupled between inductor L₁and ground for providing a large capacitance to maintain a constant DCvoltage.

Circuit 5, and more particularly L₁ and C₃, is coupled to a directcurrent ("DC") voltage source to receive a DC bias input V_(IN),typically 6 V. Transmitter circuit 5 also receives a data input signalV_(DATA) for encoding the RF carrier signal. As detailed hereinabove,circuit 5 generates a radiating output signal via inductor L₁. In doingso, transistor Q₁, acting as an amplifier, in combination with theresonating tank circuit, generates a resonating signal which is providedto inductor L₁ as an oscillating current signal I. The conduction ofcurrent I through inductor L₁ in turn causes the radiating output signalto be transmitted as an electromagnetic field.

The above described Colpitts oscillator is well suited for the RF signaltransmission applications of a remote keyless entry system. However,such an oscillator design provides a limited amount of power output.Further, the alternative of a greater inductance value for radiatinginductor L₁ may not feasibly achieve a corresponding increase in powerdue to the inherent limitations of such components. Similar attempts toenhance output power through the optimization of component values hasproved futile in view of the matching losses created thereby. Moreover,rail-to-rail voltage swings in transistor Q₁ tend to confine the amountof current flow through the circuit which, in turn, diminishes theavailable power output realized by a given transmitter circuit.

As a result of the limited power available from compact remotetransmitters using Colpitts oscillators, another problem has arisen withtheir application in compact remote transmitters. Typically, compactremote transmitters are hand grasped and directed generally toward areceiver of the system. By so doing, a parasitic impedance is created bythe user's hand. This additional impedance reduces the amount oftransmitted energy towards the receiver. This becomes an issue ofparticular significance in view of the limited power available from atraditional Colpitts oscillator.

In view of these problems, a need remains for an oscillator circuithaving an increased power output. A demand further exists for a methodof efficiently generating and transmitting an RF signal having increasedpower output. Moreover, industry requires an oscillator circuit which issubstantially immune to parasitic impedances.

SUMMARY OF THE INVENTION

The primary advantage of the present invention is to overcome thelimitations of the prior art.

Another advantage of the present invention is to provide for a balancedoscillator and transmitter having enhanced power output characteristics.

A further advantage of the present invention is to provide for abalanced oscillator and transmitter substantially immune to parasiticimpedances.

Still another advantage of the present invention is to provide for amethod of efficiently generating and transmitting an RF signal which mayrealize increased power output.

Yet still another advantage of the present invention is to provide for abalanced oscillator and transmitter circuit and method of achieving anefficient transmission capable of offering increased power output andsuitable for use with a remote vehicular keyless entry system.

In order to achieve the advantages of the present invention, anoscillator and transmitter system is disclosed. The oscillator comprisesa resonator for generating a reference signal having a resonatingfrequency, a first oscillator for providing a first oscillating outputin response to the resonating frequency, and a second oscillator forproviding a second oscillating output in response to the resonatingfrequency. The second oscillating output has a magnitude equal to thefirst oscillating output while oscillating 180 degrees out of phase withthe first oscillating output. The transmitter comprises an antenna forradiating the first and second oscillating output signals.

These and other advantages and objects will become apparent to thoseskilled in the art from the following detailed description read inconjunction with the appended claims and the drawings attached hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood from reading thefollowing description of non-limitative embodiments, with reference tothe attached drawings, wherein below:

FIG. 1 illustrates a circuit diagram illustrating a conventional singleColpitts-type oscillator and transmitter circuit;

FIG. 2 illustrates a block diagram of a balanced oscillator andtransmitter system according to a first embodiment of the presentinvention;

FIG. 3 illustrates a first circuit realization of the first embodimentof the present invention;

FIG. 4 illustrates a second circuit realization of the first embodimentof the present invention;

FIG. 5 illustrates a circuit realization of a first aspect of thepresent invention;

FIG. 6 illustrates a graphical representation of voltage waveformsachieved by the first embodiment of the present invention;

FIG. 7 illustrates a block diagram of a balanced oscillator andtransmitter system according to the preferred embodiment of the presentinvention;

FIG. 8 illustrates a preferred circuit realization of the preferredembodiment of the present invention;

FIG. 9 illustrates a further circuit realization of the preferredembodiment of the present invention; and

FIG. 10 illustrates still another alternate embodiment of the presentinvention.

It should be emphasized that the drawings of the instant application arenot to scale but are merely schematic representations and are notintended to portray the specific parameters or the structural details ofthe invention, which can be determined by one of skill in the art byexamination of the information herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 2, a balanced oscillator and transmitter system 10 isillustrated according to a first embodiment of the present invention.System 10 comprises a resonator 18 for generating a reference signalhaving a resonating frequency. Resonator 18 preferably comprises asurface acoustic wave ("SAW") device, and the resonating frequencypreferably falls within the radio frequency ("RF") spectrum. It shouldbe obvious to one of ordinary skill in the art, however, that othercomponents, such as a bulk acoustic wave ("BAW") device for example, mayalso be employed to realize the functional purpose of the resonator.

System 10 additionally comprises a first and second oscillator, 12 and15, each for generating an oscillating output in response to theresonating frequency of the resonator 18. First oscillator 12 comprisesan amplifier 14 for amplifying an input corresponding with the referencesignal provided by resonator 18, and a resonating circuit 13, coupledwith amplifier 14, for generating an oscillating signal in response tooutput of amplifier 14. Similarly, second oscillator 15 comprises anamplifier 16 for amplifying an input corresponding with the referencesignal provided by resonator 18, and a resonating circuit 17, coupledwith amplifier 16, for generating an oscillating signal in response tooutput of amplifier 16. While both oscillators preferably compriseidentical functional components, it should be apparent to one ofordinary skill in the art that alternate oscillator designs may berealized while still achieving the advantages of the present invention.To provide a balanced design, the outputs of both oscillators 12 and 15are 180 degrees out of phase with one another, yet equal in magnitude.

System 10 moreover comprises an antenna 11 for radiating an outputsignal having a single frequency. The output signal of antenna 11corresponds with the sum of both first and second oscillating outputs.The relationship between the output signal and the first and secondoscillating signals can be best understood by appreciating the outputcharacteristics of system 10. Comprising an output impedance, system 10can be viewed using a voltage divider model. Using this illustration,both first and second oscillator outputs are representative of an inputto the divider. The model further comprises a first impedance associatedwith the impedance seen by each oscillator to ground, as well as asecond impedance in series with the first .impedance. Second impedanceis a model of the output impedance of system 10. By way of this voltagedivider model, the output signal generated by antenna 11 isrepresentative of the voltage falling across the first impedance. Thus,in view of its balanced characteristics, the output signal transmittedby antenna 11 of system 10 differs from the sum of the oscillatingoutputs in amplitude alone, though the current is the same. It is,nonetheless, conceivable that the output signal might be intentionallydistinguishable from the sum of the oscillating outputs in frequency orphase, as well as a combination thereof, for example, as would beapparent to one of ordinary skill in the art.

Antenna 11 preferably comprises an inductor having a direct current("DC") center point. This DC center point partitions the inductor into afirst and second equivalent inductors. Furthermore, antenna 11 comprisesan alternating current ("AC") balanced oscillating point which providesa location along antenna 11 where the AC voltage magnitude of theoscillating outputs of first and second oscillators 12 and 15 are bothsubstantially zero. In view of both the AC and DC center points, a"balanced" oscillator is realized.

Tight tolerances for resonating circuits 13 and 17 are not required forthe present balanced oscillator design. This benefit is achieved by wayof the DC center point and the AC center point, as well as the balancedcircuit itself. Moreover, as antenna 11 preferably transmits bothoscillator outputs at a single primary frequency, the tolerancesassociated with resonating circuits 13 and 17 are less critical to theoverall operation of system 10.

In a further embodiment of the present invention, antenna 11 comprises aprimary winding of a center tapped transformer for transmitting theoscillating outputs of both first and second oscillators 12 and 15 ontoa secondary winding. By this arrangement, secondary winding may act asantenna itself by radiating the oscillating outputs. However, thisapproach is preferred for low frequency operation. To support operationat other frequencies, an output inductor or the like should be employedin conjunction with a filter and matching circuit to radiate theoscillating outputs.

Referring to FIG. 3, a circuit realization 20 is depicted of thebalanced oscillator and transmitter system of FIG. 2. Balancedoscillator and transmitter circuit 20 comprises a first and secondpseudo Colpitts oscillator. Both pseudo Colpitts oscillators arebalanced with respect to one another and share a common tank circuit andoscillating current signal I for power output efficiency. Circuit 20described herein is particularly applicable with automotive remotekeyless entry systems. Other applications, however, are clearlyconceivable to one of ordinary skill in the art.

According to a more detailed description, circuit 20 comprises abalanced oscillator configuration which includes two pseudo Colpittsoscillator circuits for producing a local oscillation signal. Theoscillator circuitry includes a first transistor Q₂ and a secondtransistor Q₃ each coupled with a resonator device 22 therebetween.Resonator device 22 acts as a series resonant input tank for generatingand stabilizing the oscillating current signal I. By so doing, aresonance RF carrier frequency is achieved.

First and second transistors, Q₂ and Q₃, each preferably comprise abipolar junction transistor ("BJT"). Alternatives, however, such as aheterojunction bipolar transistor ("HBT"), should be apparent to one ofordinary skill in the art. According to a further embodiment,transistors Q₂ and Q₃ are each MMBTH10 type bipolar transistors.

Transistors Q₂ and Q₃ each operate as an amplification stage to providea unity loop gain for steady state operations. First transistor Q₂comprises a base, a collector, and emitter 30, 32 and 34, respectively.Likewise, second transistor Q₃ comprises a base, a collector, andemitter 36, 38 and 40, respectively. Transistors Q₂ and Q₃ are eachconfigured as a pseudo Colpitts Oscillator having a tuned LC circuitryand positive feedback. It should be understood by one of ordinary skillin the art that various other transistor oscillator configurations maybe substituted into the above arrangement to achieve the same functionalpurpose.

Resonator device 22 is coupled between the base terminals 30 and 36 oftransistors Q₂ and Q₃ via resonator output lines 42 and 44,respectively. Resonator 22 is shown having an array of metallic fingersformed on a piezoelectric substrate. Resonator 22 advantageouslyoperates to stabilize oscillations of the carrier signal. Resonatordevice 22 preferably comprises a series resonant input tank circuitsurface acoustic wave ("SAW") device. However, according to a furtherembodiment, SAW resonator 22 is a RO2073 SAW resonator manufactured andsold by RF Monolithics, Incorporated.

Circuit 20 further comprises a pair of output tank circuits. Each outputtank circuit includes a capacitor and inductor; first input tankcomprises first inductor L₂ and second input tank comprises secondinductor L₃. Inductors L₂ and L₃ each operate as antenna radiatingelements for radiating an output signal in response to the commonlyshared oscillating current signal I. First inductor L₂ is coupledbetween collector terminal 32 of transistor Q₂ and node 28, while secondinductor L₃ is coupled between collector terminal 38 of transistor Q₃and node 28. Accordingly, inductors L₂ and L₃ are coupled together atnode 28 in a series connection. A voltage input source 24 is coupled tonode 28 between inductors L₂ and L₃ for applying a DC voltage inputV_(IN) thereto. According to one example of the present invention,voltage input signal V_(IN) is a +3 volt DC signal. Application of the+3 volts between inductors L₂ and L₃ biases transistors Q₂ and Q₃ torealize the necessary gain. Inductors L₂ and L₃ each operate as anantenna for transmitting and radiating an electromagnetic fieldexhibiting the oscillating signal with the predetermined carrierfrequency.

Circuit 20 further comprises a data input 26 coupled to both resonatoroutput lines 42 and 44 though respective resistors R₆ and R₇. Data input26 is adapted to receive an on/off data input signal V_(DATA) which isapplied to both sides of SAW resonator 22. Each of the resonator outputlines 42 and 44 is also coupled to ground via respective resistors R₅and R₈. The data input signal V_(DATA) encodes the carrier signal with amodulation scheme to provide information on the carrier signal. Thepreferred modulation format is amplitude modulation ("AM"), though pulsewidth modulation for example, and others may be easily substituted byone of ordinary skill in the art. The information provided on thecarrier signal may control and/or initiate various system operations,such as a door lock actuation mechanism, as well as the on/offoperations of circuit 20. Application of data input signal V_(DATA) maybe initiated by manual control through an actuation mechanism such as,for example, a push-button pad, switch or other pulsed activationdevice.

SAW resonator 22 provides for an input tank circuit which is commonlyshared by the pair of pseudo Colpitts. Inductor L₂, in combination withcapacitors C₄ and C₅, furnishes a first output tank circuit. Similarly,inductor L₃, in combination with capacitors C₆ and C₇, creates a secondoutput tank circuit. While the series resonant input tank stabilizesoscillation of the resonating signal, the output tanks provide forradiation of the RF output signal. Capacitors C₄ and C₅ also establish avoltage divider network, as well as a positive feedback path totransistor Q₂. Likewise, capacitors C₆ and C₇ creates a voltage dividerand a positive feedback path to transistor Q₃. Energy is efficientlystored in the capacitors C₄ through C₇ and inductors L₂ and L₃ toenhance radiation efficacy by reducing the amount of energy that mayotherwise be required for each cycle of transistors Q₂ and Q₃.

Referring to FIG. 4, circuit 20 may alternately be configured to includea center-tapped transformer 46 in lieu of first and second inductors L₂and L₃. To this end, center-tapped transformer 46 comprises a primarywinding having a first primary winding portion 48a and second primarywinding portion 48b. Primary winding portions 48a and 48b preferably areof substantially equal size. The voltage input source 24 is coupled to acenter tap 49, located between the primary winding portions 48a and 48b,for supplying DC voltage input V_(IN) thereto.

Center-tapped transformer 46 further comprises a secondary winding 50located adjacent to the primary winding portions 48a and 48b.Transformer 46 is adapted to form a first magnetic coupling betweenprimary winding portion 48a and the secondary winding 50, and a secondmagnetic coupling between primary winding portion 48b and secondarywinding 50. The secondary winding 50 in turn is coupled on both ends toa filter and matching network 52. A pair of output lines extending fromthe filter and matching network 52 are coupled to a radiating inductorL₄ for radiating an output electromagnetic field therefrom.

According to the alternate embodiment of FIG. 4, the first and secondprimary winding portions 48a and 48b of the center-tapped transformer 46each produce an electromagnetic field in response to the oscillatingcurrent signal I that is transmitted therethrough. The electromagneticfields from each of primary winding portions 48a and 48b are therebytransmitted and induced onto the secondary winding 50 of thecenter-tapped transformer 46. The signals induced onto secondary winding50 are summed together. The summed signal is in turn filtered toeliminate undesirable noise, and is impedance matched via filter andmatching network 52. The filtered and impedance matched signal is thenpassed through a radiating inductor L₄ to transmit a single radiatingoutput signal. Use of the center-tapped transformer 46 advantageouslyseparates out the even harmonics and is generally better able to achieveenhanced control of the transmission of the single radiating outputsignal.

It should be understood that the SAW resonator 22 is a series-resonantinput tank circuit which may be implemented with alternate comparableseries resonant frequency stabilizing devices. As an alternative to theSAW resonator 22, the series resonant tank circuit may include a bulkacoustic wave ("BAW") device, crystal device, microstrip or any otherseries-resonant structure or device that may achieve the desiredstabilizing signal oscillation.

With particular reference to FIG. 5, a series resonant tank circuit 60is depicted as an alternative to the SAW resonator 22 of FIGS. 2-4.Here, series resonant tank circuit 60 comprises a resistor R_(M),capacitor C_(M) and inductor L_(M). Each of these components are coupledin series to create series resonant tank circuit 60. The resonantfrequency of the tank circuit 60 is generally dependant on the size ofthe inductor L_(M) and capacitor C_(M).

In operation, circuit 20 receives a DC input voltage signal V_(IN)through voltage input source 24. Data input V_(DATA) may also bereceived via data input 26 to encode the carrier signal with apredetermined modulation scheme. Initially, circuit 20 forms aresonating signal which starts up and builds to a steady state energylevel having oscillations at a known frequency. In doing so, transistorsQ₂ and Q₃ cycle between the collector terminal 38 and emitter terminal40 in response to noise or other induced signals and will build untilthe steady state is reached.

During start up, each amplification stage provides a gain in excess ofunity. At steady state, the gain of each amplification stage isapproximately equal to or slightly greater than unity to account for anyenergy loss. The series resonant tank circuit with SAW resonator 22maintains and ensures the stability of the signal oscillation within thecircuit 20. The oscillating signal in turn is exhibited by currentsignal I flowing through the antenna radiating elements, inductors L₂and L₃. In addition, the feedback paths provided via capacitors C₄ andC₅ and capacitors C₆ and C₇ create a phase delay which adjusts the looptime to realize the desired frequency.

Referring to FIG. 6, a graphical representation of voltage waveformsachieved by the first embodiment of the present invention is depicted.Here, the inductors L₂ and L₃ of circuit 20 of FIG. 2 each radiate aseparate signal through separate electromagnetic fields, both of whichhave the same carrier frequency in response to the commonly sharedoscillating current signal I. These radiating output signals frominductors L₂ and L₃ and the total summed radiating output areillustrated by the waveforms 66 provided in FIG. 6. The first radiatingoutput signal transmitted from inductor L₂ is shown as voltage waveform62, while the second radiating output signal transmitted from inductorL₃ is depicted as voltage waveform 64. Voltage waveforms 62 and 64 arecharacterized as having equal amplitudes and an approximate 180 degreephase shift relationship relative to one another. Radiating signals 62and 64 emitted are measured with respect to voltage node 28 andtherefore exhibit the aforementioned phase shift of 180 degrees. Aswaveforms 62 and 64 are both measured relative to node 28, the summationof both waveforms 62 and 64 relative to the commonly shared currentsignal I results in a voltage waveform representing a single radiatingoutput signal 66. Accordingly, Output signal 66 may be achieved usingthe pair of balanced oscillators and output tanks of the presentinvention.

Single radiating output signal 66 in one embodiment has a frequency ofapproximately 315 MHz. Additionally, the outputs from both inductor L₂and inductor L₃ of the first and second output tanks are balancedsignals which are symmetrical relative to node 28 which is preferablyset at +3 volts DC. In contrast, the separate radiating signals createdby center-tapped transformer 46 of one of the alternate embodiments ofthe present invention, may be summed and then filtered and impedancematched prior to transmission.

Referring to FIG. 7, a buffered balanced oscillator and transmittersystem 70 is illustrated according to the preferred embodiment of thepresent invention. System 70 comprises a resonator 72 for generating areference signal having a resonating frequency. Resonator 72 preferablycomprises a surface acoustic wave ("SAW") device, and the resonatingfrequency preferably falls within the radio frequency ("RF") spectrum.It should be obvious to one of ordinary skill in the art, however, thatother components, such as a bulk acoustic wave ("BAW") device forexample, may also be employed to realize the functional purpose of theresonator.

System 70 additionally comprises a first and second oscillator, 74 and76, each for generating an oscillating output in response to theresonating frequency of the resonator 72. First oscillator 74 comprisesan amplifier 78 for amplifying an input corresponding with the referencesignal generated by resonator 72, and a resonating circuit 80, coupledwith amplifier 78, for generating an oscillating signal in response tooutput of amplifier 78. Similarly, second oscillator 76 comprises anamplifier 82 for amplifying an input corresponding with the referencesignal generated by resonator 72, and a resonating circuit 84, coupledwith amplifier 82, for generating an oscillating signal in response tooutput of amplifier 82. While both oscillators preferably compriseidentical functional components, it should be apparent to one ofordinary skill in the art that alternate oscillator designs may berealized while still achieving the advantages of the present invention.To provide a balanced design, the outputs of both oscillators 74 and 76are 180 degrees out of phase with one another, yet equal in magnitude.

As a means to substantially minimize the effects of parasiticimpedances, such as those created by a user's hand in holding or cuppinga compact remote RF transmitter during operation, system 70 alsocomprises a first and second buffer, 86 and 88. First and second buffer,86 and 88, functionally isolate resonating circuits 80 and 84,respectively, from an antenna 100. To this end, first buffer 86comprises a buffer amplifier 90 which is coupled with a resonatingcircuit 93, while second buffer amplifier 88 comprises a bufferamplifier 96 also coupled with resonating circuit 93. Resonating circuit93 comprises a series or parallel tuned resonant tank and a radiatingelement 100. By this arrangement, the output impedance of system 70, asviewed from the DC center point of antenna 100 along both paths createdby oscillators 74 and 76, is substantially decreased, and the current isincreased. This decrease in impedance and increase in current by way ofcurrent gain result in the output signal, as radiated by antenna 100,comprising a larger percentage of the first and second oscillatingsignals. With the output signal having a larger percentage of the firstand second oscillating signals, a more powerful output signal, and assuch, a more powerful transmitter is realized.

Moreover, system 70 comprises antenna 100 for radiating an output signalhaving a single frequency. The output signal of antenna 100 correspondswith the sum of both first and second oscillating outputs. Therelationship between the output signal and the first and secondoscillating signals can be best understood by appreciating the outputcharacteristics of system 70. Comprising an output impedance, system 70can be viewed using a voltage divider model. Using this illustration,both first and second oscillator outputs are representative of an inputto the divider. The model further comprises a first impedance associatedwith the impedance as seen by each oscillator to ground, as well as asecond impedance in series with the first impedance. Second impedance isa model of the output impedance of system 70. By way of this voltagedivider model, the output signal generated by antenna 100 isrepresentative of the voltage falling across the first impedance. Thus,in view of its balanced characteristics, the output signal transmittedby antenna 100 of system 70 differs from the sum of the oscillatingoutputs in amplitude alone. Nonetheless, it is conceivable that theoutput signal might be intentionally distinguishable from the sum of theoscillating outputs in frequency or phase, as well as a combinationthereof, for example, as would be apparent to one of ordinary skill inthe art.

Antenna 100 preferably comprises an inductor, as part of resonantcircuit 93, having a direct current ("DC") center point. This DC centerpoint partitions the inductor into a first and second equivalentinductors. From this center point, a high impedance is created to groundthrough each oscillator. Furthermore, antenna 100 comprises analternating current ("AC") balanced oscillating point which provides alocation along antenna 100 where magnitude of the oscillating outputs offirst and second oscillators 74 and 76 are both substantially zero. Inview of both the AC and DC center points, a "balanced" oscillator isrealized.

Tight tolerances for resonating circuits 80, 84 and 93 are not requiredfor the present balanced oscillator design. This benefit is achieved byway of the DC and AC center points, as well as the balanced circuititself. Moreover, as antenna 100 preferably transmits both oscillatoroutputs at a single primary frequency, the tolerances associated withresonating circuits 80, 84 and 93 are less critical to the overalloperation of system 70.

In a further embodiment of the present invention, antenna 100 comprisesa primary winding of a center tapped transformer for transmitting theoscillating outputs of both first and second oscillators 74 and 76 ontoa secondary winding. By this arrangement, secondary winding may itselfradiate the oscillating outputs. In the alternative, an output inductoror the like may be employed in conjunction with a filter and matchingcircuit for the purpose of radiating the oscillating outputs.

In still another embodiment of the present invention, a device is alsoincorporated for increasing the output range of each of the first andsecond oscillating output signals. Using aforementioned voltage dividermodel, this device further and more directly increases the output powerto reduce the sensitivity of system 70 to the parasitic impedancesdefined above. The device preferably comprises a capacitor, though otherimpedances may be used, which applies a greater amount of the voltagerange associated with amplifiers 78 and 82 to buffer amplifiers 90 and96, respectively.

Referring to FIG. 8, a preferred circuit realization 110 of the bufferedbalanced oscillator and transmitter system of FIG. 7 is illustrated.Buffered balanced oscillator and transmitter circuit 110 comprises afirst and second pseudo Colpitts oscillator. Both pseudo Colpittsoscillators are balanced with respect to one another and share a commontank circuit and oscillating current signal I for power outputefficiency. Circuit 110 described herein is particularly applicable withautomotive remote keyless entry systems. Other applications, however,are clearly conceivable to one of ordinary skill in the art.

According to a more detailed description, circuit 110 comprises abalanced oscillator configuration which includes the two pseudo Colpittsoscillator circuits for producing a local oscillation signal. Theoscillator circuitry includes a first transistor Q₄ and a secondtransistor Q₅ each coupled with a resonator device 112 therebetween.Resonator device 112 acts as a series resonant input tank for generatingand stabilizing the oscillating current signal I. By so doing, aresonance RF carrier frequency is achieved.

First and second transistors, Q₄ and Q₅, each preferably comprise abipolar junction transistor ("BJT"). Alternatives, however, such as aheterojunction bipolar transistor ("HBT"), should be apparent to one ofordinary skill in the art. According to a further embodiment,transistors Q₄ and Q₅ are each MMBTH10 type bipolar transistors.

Transistors Q₄ and Q₅ each operate as an amplification stage to providea unity loop gain for steady state operations. First transistor Q₄comprises a base, a collector, and emitter 120, 122 and 124,respectively. Likewise, second transistor Q₅ comprises a base, acollector, and emitter 126, 128 and 130, respectively. Transistors Q₄and Q₅ are each configured as a pseudo Colpitts oscillator having atuned LC circuitry and positive feedback. It should be understood by oneof ordinary skill in the art that various other transistor oscillatorconfigurations may be substituted into the above arrangement to achievethe same functional purpose.

Resonator device 112 is coupled between the base terminals 120 and 126of transistors Q₄ and Q₅ via output lines 132 and 134, respectively.Resonator 112 is shown having an array of metallic fingers formed on apiezoelectric substrate. Resonator 112 advantageously operates tostabilize oscillations of the carrier signal. Resonator device 112preferably comprises a series resonant input tank circuit surfaceacoustic wave ("SAW") device. However, according to a furtherembodiment, SAW resonator 112 is a RO2073 SAW resonator manufactured andsold by RF Monolithics, Incorporated.

Circuit 110 further comprises a pair of output tank circuits, which incombination with transistors Q₄ and Q₅ form a first and secondoscillator. Each output tank circuit includes a capacitor and inductor;first input tank comprises first inductor L₅ and second input tankcomprises second inductor L₆. First inductor L₅ is coupled betweencollector terminal 122 of transistor Q₂ and node 118, while secondinductor L₆ is coupled between collector terminal 128 of transistor Q₅and node 118. Accordingly, inductors L₅ and L₆ are coupled together atnode 118 in a series connection. A voltage input source 114 is coupledto node 118 between inductors L₅ and L₆ for applying a DC voltage inputV_(IN) thereto. According to one example of the present invention,voltage input signal V_(IN) is a +3 volt DC signal. Application of the+3 volts between inductors L₄ and L₅ biases transistors Q₄ and Q₅ torealize the necessary gain.

Circuit 110 further comprises a data input 116 coupled to both resonatoroutput lines 132 and 134 though respective resistors R₁₂ and R₁₃. Datainput 116 is adapted to receive an on/off data input signal V_(DATA)which is applied to both sides of SAW resonator 112. Each of theresonator output lines 132 and 134 is also coupled to ground viarespective resistors R₁₁ and R₁₄. The data input signal V_(DATA) encodesthe carrier signal with a modulation scheme to provide information onthe carrier signal. The preferred modulation format is amplitudemodulation ("AM"), though pulse width modulation for example, and othersmay be easily substituted by one of ordinary skill in the art. Theinformation provided on the carrier signal may control and/or initiatevarious system operations, such as a door lock actuation mechanism, aswell as the on/off operations of circuit 20. Application of data inputsignal V_(DATA) may be initiated by manual control through an actuationmechanism such as, for example, a push-button pad, switch or otherpulsed activation device.

SAW resonator 112 provides for an input tank circuit which is commonlyshared by the pair of balanced oscillators. Inductor L₅, in combinationwith capacitors C₈ and C₉, furnishes a first output tank circuit.Similarly, inductor L₆, in combination with capacitors C₁₀ and C₁₁,creates a second output tank circuit. While the series resonant inputtank stabilizes oscillation of the resonating signal, the output tanksprovide for radiation of the RF output signal. Capacitors C₈ and C₉ alsoestablish a voltage divider network, as well as a positive feedback pathto transistor Q₄. Likewise, capacitors C₁₀ and C₁₁ create a voltagedivider and a positive feedback path to transistor Q₅. Energy isefficiently stored in the capacitors C₈ through C₁₁ and inductors L₅ andL₆ to enhance radiation efficacy by the antenna so as to reduce theamount of energy that may otherwise be required for each cycle oftransistors Q₄ and Q₅.

Antenna 100 of FIG. 7 is realized in the present embodiment by inductorL₇ for transmitting and radiating an electromagnetic field exhibitingthe buffered oscillating signal with the predetermined carrierfrequency. Inductor L₇, in further embodiments, may additionallycomprise two inductors coupled together in series having a center pointwith a common DC feed, as well as a center point without a common DCfeed comprising a resistor juxtapositioned between both inductorsrunning to ground.

Circuit 110 also provides means to substantially minimize the effects ofparasitic impedances. To realize this aspect of the invention, depictedas first and second buffer, 86 and 88 in FIG. 7, circuit 110 furthercomprises a third transistor Q₆ and a fourth transistor Q₇. Third andfourth transistors, Q₆ and Q₇, both preferably comprise a bipolarjunction transistor. Alternatives, however, such as a heterojunctionbipolar transistor ("HBT"), are available and should be apparent to oneof ordinary skill in the art. According to a further embodiment,transistors Q₆ and Q₇ are each MMBTH10 type bipolar transistors.

Transistors Q₆ and Q₇ each operate as a buffer for buffering the firstand second oscillating output signals generated by their respectivepseudo Colpitts oscillator. Transistor Q₆ is coupled with both a firstsupplemental tank and the output resonant tank associated withtransistor Q₄, while transistor Q₇ is coupled with both a secondsupplemental resonant tank and the output resonant tank associated withtransistor Q₅. First and second supplemental resonant tanks, referred toas resonating circuit 93 in FIG. 7, functionally decrease the outputimpedance of circuit 110. By so doing, the output signal ultimatelyradiated by the antenna L₇ comprises an increased percentage of thefirst and second output signals.

Transistor Q₆ comprises a base, collector and emitter, 136, 138 and 140,respectively, while transistor Q₇ comprises a base, collector andemitter, 142, 144 and 146, respectively. The base 136 of transistor Q₆is coupled between capacitors C₈ and C₉, at the node for which emitter124 of transistor Q₄ also is coupled with resistor R₁₀, while collector138 is coupled with node 118. Further, emitter 140 is coupled with thefirst supplemental resonant tank. First supplemental resonant tankcomprises capacitor C₁₂ and resistor R₁₆, which are both grounded, aswell as a first terminal of inductor L₇, which is coupled with thesecond supplemental resonant tank. Similarly, the base 142 of transistorQ₇ is coupled between capacitors C₁₀ and C₁₁, at the node for whichemitter 130 of transistor Q₅ also is coupled with resistor R₁₅, whilecollector 144 is coupled with node 118. Further, emitter 146 is coupledwith the second supplemental resonant tank. Second supplemental resonanttank comprises capacitor C₁₃ and resistor R₁₇, which are both grounded,as well as the second terminal of inductor L₇, which is coupled with thefirst supplemental resonant tank. It should be understood by one ofordinary skill in the art that various other transistor bufferconfigurations may be substituted into the above arrangement to achievethe same functional purpose.

Referring to FIG. 9, a second circuit realization of the preferredembodiment of the present invention is illustrated. Circuit 110 of FIG.8 may alternately be configured to include a device for increasing theoutput range of each of the oscillating outputs of the first and secondpseudo Colpitts oscillators. This device preferably comprises a firstand second voltage divider circuit for first and second pseudo Colpittsoscillators, respectively.

According to a more detailed description, transistors Q₄ and Q₅ are eachcoupled with transistors Q₆ and Q₇, respectively, by means of a firstand second modified resonant circuit forming voltage divider circuits.With respect to transistor Q₄, collector 122 is coupled with capacitorC₈, while emitter 124 is coupled with capacitors C₉ and C₁₄, as well asresistor R₁₀. Moreover, capacitor C₈ is coupled with capacitor C₁₄ at aninput node to transistor Q₆. Similarly, collector 128 of transistor Q₅is coupled with capacitor C₁₀. Emitter 130 is coupled with capacitorsC₁₁ and C₁₅, and resistor R₁₅. Capacitor C₁₀ is also coupled withcapacitor C₁₅ at an input node to transistor Q₇. The bases 136 and 142of transistors Q₆ and Q₇ are fed by transistors Q₄ and Q₅ at the pointwhere capacitors C₈ and C₁₄, as well as C₁₀ and C₁₅, respectively coupletogether to generate a greater voltage swing.

Furthermore, a resistor network is provided between V_(DATA) and themodified resonant circuit, detailed herein. With respect to transistorQ₄, resistor R₁₉ is coupled with the input node of the base 136 oftransistor Q₆, while resistor R₁₈ is coupled from the input node of base136 to ground. As such, resistor R₁₈ is in parallel with capacitors C₁₄and C₉. Likewise, resistor R₂₀ is coupled with the input node of base142 of transistor Q₇, and resistor R₂₁ is coupled from this input nodeof base 142 to ground such that resistor R₂₁ is positioned in parallelwith capacitors C₁₅ and C₁₁. In so doing, the output ranges createdacross resistors R₁₈ and R₂₁ are substantially increased. This increaseis attributable to the repositioning of bases 136 and 142 with thecollector to ground voltages of transistors Q₄ and Q₅, respectively, andtheir associated ranges, in view of the added voltage divider. In oneembodiment, using certain values for the above capacitor and resistorcomponents, the voltage range is increased by 100 percent.

Referring to FIG. 10, a further alternate embodiment depicting abuffered oscillator and transmitter circuit 160 is illustrated. Circuit160 comprises three functional stages: a pseudo Colpitts oscillator 162,a buffer 164 and an output system 166. Circuit 160 described herein isparticularly applicable with automotive remote keyless entry systems.Other applications, however, are clearly foreseeable to one of ordinaryskill in the art.

According to a more detailed description, oscillator 162 comprises aColpitts configured transistor Q₁₀ and an input resonant tank circuit.The tank circuit typically comprises a resonator, such as a surfaceacoustic wave ("SAW") device 172, a pair of feedback capacitors, C₁₆ andC₁₇, an inductor L₈, as well as a capacitor C₁₉ for providing a largecapacitance to maintain a constant DC voltage. Further, the oscillatoralso includes a number of biasing resistors to facilitate the properoperation of transistor Q₁₀. Transistor Q₁₀ functionally provides aunity loop gain for steady state operations.

Structurally, transistor Q₁₀ comprises a base 176, collector 178 and anemitter 180. Base terminal 176 is coupled with surface acoustic waveresonator 172, and collector 178 is coupled with inductor L₈, whileemitter 180 is coupled to ground through a resistor R₂₄. Additionally,feedback capacitor C₁₆ is coupled between emitter 180 and ground, and assuch, is in parallel with resistor R₂₄, while feedback capacitor C₁₇ iscoupled between collector 178 and emitter 180. Capacitor C₁₉ is coupledbetween ground and V_(IN).

Transistor Q₁₀ is coupled to a direct current ("DC") voltage source 170through inductor L₈ to receive a DC bias input V_(IN), typically 6 V.Oscillator 162 also receives a data input signal V_(DATA) 168 forencoding the RF carrier signal, by means of a resistor network forming avoltage divider circuit. Data input 168 is adapted to receive an on/offdata input signal V_(DATA) which is applied to SAW resonator 172. Thedata input signal V_(DATA) encodes the carrier signal with a modulationscheme to provide information on the carrier signal. The preferredmodulation format is amplitude modulation ("AM"), though pulse widthmodulation for example, and others may be easily substituted by one ofordinary skill in the art. The information provided on the carriersignal may control and/or initiate various system operations, such as adoor lock actuation mechanism, as well as the on/off operations ofcircuit 160. Application of data input signal V_(DATA) may be initiatedby manual control through an actuation mechanism such as, for example, apush-button pad, switch or other pulsed activation device. By thisconfiguration, transistor Q₁₀, acting as an amplifier, in combinationwith the resonating tank circuit, generates an oscillating outputsignal.

Transistors, Q₁₀ and Q₁₁, each preferably comprise a bipolar junctiontransistor ("BJT"). Alternatives, however, such as a heterojunctionbipolar transistor ("HBT"), should be apparent to one of ordinary skillin the art. According to a further embodiment, transistors Q₂ and Q₃ areeach MMBTH10 type bipolar transistors.

Resonator device 172 is coupled between base 176 of transistor Q₁₀ andground. Resonator 172 advantageously operates to stabilize oscillationsof the carrier signal. Resonator device 172 preferably comprises aseries resonant input tank circuit surface acoustic wave ("SAW") device.However, according to a further embodiment, SAW resonator 22 is a RO2073SAW resonator manufactured and gold by RF Monolithics, Incorporated.

Buffer 164 functionally minimizes the effects of parasitic impedancescreated through various means detailed herein. To realize this benefit,buffer 164 comprises a transistor Q₁₁, as well as a buffer resonant tankof inductor L₉ and capacitor C₁₈. Transistor Q₁₁ comprises a base 184, acollector 186 and an emitter 182. Buffer 164 is coupled with oscillator162 at two nodes. First, buffer 164 receives a DC bias input V_(IN)through direct current ("DC") voltage source 170 at collector 186,wherefrom L₈ of oscillator 162 is also biased. Buffer 164 is alsocoupled with oscillator 162 at emitter 180 of transistor Q₁₀ and base184 of transistor Q₁₁.

Output stage 166 is coupled with buffer 164 for the purpose oftransmitting the oscillating signal. The output of buffer 164, having anoscillating output at the resonant frequency, is transmitted across tostage 166. Stage 166 additionally comprises a device 174 for matchingthe output impedance of the circuit. Finally, output stage 166 comprisesan antenna in the form of inductor L₁₀ for transmitting the resultantoscillating signal.

It should be noted that the oscillator and transmitter circuits of thepresent invention may be mounted within a compact enclosure andadvantageously employed to transmit control signals, especially for usein connection with a remote controlled keyless entry system. For such anapplication, the user may manually activate the V_(DATA) input to encodethe carrier signal with selected information. The carrier signal andmodulating information are then radiated from the transmitter circuitsby means of the output tanks. A receiver which is generally mountedwithin a vehicle will receive the radiating signal, decode themodulating information and initiate and/or execute the selectedoperation such as locking or unlocking a vehicle door, activating ordeactivating an alarm system, for example. In contrast to conventionalapproaches, these circuits advantageously achieve increased output powerand maintain an efficient power usage therewith.

Furthermore, it should also be apparent that the embodiments of thepresent invention may use various sized components which may be modifiedwithout departing from the invention. As one example, inductors L₈ andL₉ each provide an inductance of approximately 40 nH. Capacitors C₁₇ andC₁₈ each may have a capacitance of approximately 4.7 pF, while capacitorC₁₆ has a capacitance of about 22 pF. Resistor R₂₃ may have a resistanceof about 15 kΩ. Resistor R₂₂ may have a resistance of about 6.8 kΩ,while resistor R₂₄ has a resistance of about 180 kΩ.

While the particular invention has been described with reference toillustrative embodiments, this description is not meant to be construedin a limiting sense. It is understood that although the presentinvention has been described in a preferred embodiment, variousmodifications of the illustrative embodiments, as well as additionalembodiments of the invention, will be apparent to persons skilled in theart upon reference to this description without departing from the spiritof the invention, as recited in the claims appended hereto. Thus, forexample, it should be apparent to one of ordinary skill in the art thatwhile the transmitter herein has been detailed as operating in the RFfrequency range, other formats are available which would take fulladvantage of the present invention. Similarly, while bipolar junctiontransistors are described herein as one potential realization of anamplifier, other designs are available which utilize other transistortypes, such as field effect transistors ("FETs"), JFETs and MOSFETs, forexample, known to one of ordinary skill in the art. Moreover, theantenna of the present invention may also be realized by a patch antennadesign, as would be apparent to one of ordinary skill in the art in viewof the present invention. It is therefore contemplated that the appendedclaims will cover any such modifications or embodiments as fall withinthe true scope of the invention.

What is claimed is:
 1. A transmitter for transmitting an output signalhaving a single frequency, the transmitter having an output impedance,an antenna for radiating the output signal corresponding with a firstand second oscillating output, and a balanced oscillator for generatingthe first and second oscillator outputs, said balanced oscillatorcomprising:a resonator for generating a reference signal; a firstoscillator for providing the first oscillating output in response to thereference signal; and a second oscillator for providing the secondoscillating output in response to said reference signal.
 2. Theinvention of claim 1, wherein the second oscillating output comprises amagnitude equal to the first oscillating output, while the secondoscillating output oscillates 180 degrees out of phase with the firstoscillating output.
 3. The invention of claim 1, wherein said resonatorcomprises a surface acoustic wave ("SAW") device.
 4. The invention ofclaim 1, wherein at least one of said first and second oscillatorscomprises:a first amplifier for amplifying said reference signal; and afirst resonating circuit for generating said oscillating output of saidat least one of said oscillators in response to said amplified referencesignal.
 5. The invention of claim 1, wherein the antenna comprises aninductor for radiating said output signal, the inductor comprising:adirect current ("DC") center point for receiving a DC bias voltage; andan alternating current ("AC") balanced oscillating point where themagnitudes of the first and second oscillating output signals aresubstantially zero.
 6. The invention of claim 5, wherein said DC centerpoint partitions the inductor into equivalent first and secondpartitioned inductors.
 7. The invention of claim 1, wherein the antennacomprises a primary winding of a center-tapped transformer fortransmitting the first and second oscillating output signals onto asecondary winding.
 8. The invention of claim 7, wherein the antennafurther comprises a radiating element for radiating a final oscillatingsignal in response to the first and second oscillating output signalstransmitted onto said secondary winding.
 9. The invention of claim 8,further comprising:a filter for removing noise from the first and secondoscillating output signals transmitted onto said secondary winding; andmatching circuit for matching the output impedance.
 10. The invention ofclaim 1, further comprising:a buffer amplifier for buffering the firstand second oscillating output signals such that the effects of aparasitic impedance are substantially minimized.
 11. The invention ofclaim 10, wherein said second amplifier comprises a second resonatingcircuit for decreasing the output impedance such that the output signalcomprises an increased percentage of the first and second oscillatingoutput signals.
 12. The invention of claim 11, further comprising adevice for increasing an output range of each of the first and secondoscillating output signals.
 13. A transmitter circuit for transmittingan output signal having a singular frequency, said transmitter circuithaving an output impedance and comprising:a balanced oscillatorcomprising:a resonator for generating a reference signal; a first and asecond amplifier, said amplifiers being coupled with said resonator; afirst and a second resonant output tank for generating a first and asecond oscillating output signal, respectively, said first outputresonant tank being coupled with said first amplifier and said secondresonant output tank being coupled with said second amplifier, saidsecond oscillating output having a magnitude equal to said firstoscillating output and oscillating 180 degrees out of phase with saidfirst oscillating output; and an antenna for radiating the output signalcorresponding with said first and second oscillating output signals. 14.The invention of claim 13, wherein said resonator comprises a surfaceacoustic wave ("SAW") device.
 15. The invention of claim 13, wherein atleast one of said first and second amplifiers comprises a bipolarjunction transistor.
 16. The invention of claim 13, wherein at least oneof said first and second output resonant tanks comprises a capacitor andan inductor.
 17. The invention of claim 13, further comprising:a firstpositive feedback path coupled with said first amplifier; and a secondpositive feedback path coupled with said second amplifier.
 18. Theinvention of claim 13, wherein said antenna comprises an inductor forradiating the output signal, said inductor comprising:a direct current("DC") center point for receiving a DC bias voltage and for partitioningsaid inductor into equivalent first and second partitioned inductors;and an alternating current ("AC") balanced oscillating point where themagnitudes of said first and second oscillating output signals aresubstantially zero.
 19. The invention of claim 13, wherein said antennacomprises:a primary winding of a center-tapped transformer fortransmitting said first and second oscillating output signals onto asecondary winding; and a radiating element, coupled with said secondarywinding, for radiating the output signal in response to said first andsecond oscillating output signals transmitted onto said secondarywinding.
 20. The invention of claim 13, further comprising:a filter forremoving noise from said first and second oscillating output signals;and a matching circuit for matching the output impedance.
 21. Theinvention of claim 13, further comprising a third and fourth amplifierfor buffering said first and second oscillating output signals,respectively, such that the effects of a parasitic impedance aresubstantially minimized.
 22. The invention of claim 21, wherein at leastone of said third and fourth amplifiers comprises a bipolar junctiontransistor.
 23. The invention of claim 21, wherein at least one of saidthird and fourth amplifiers comprises a supplemental resonant tank fordecreasing the output impedance such that the output signal comprises anincreased percentage of said first and second oscillating outputsignals.
 24. The invention of claim 23, wherein said supplementalresonant tank comprises a capacitor and an inductor.
 25. The inventionof claim 13, further comprising a device for increasing an output rangeof each of said first and second oscillating output signals.
 26. Theinvention of claim 25, wherein said device comprises a first impedance,said first impedance being coupled with the output impedance to form avoltage divider circuit.